Switched-mode power supplies currently equip the majority of televisions and monitors which include a cathode-ray tube. These power supplies operate by chopping the current.
The principle of power supplies of this type is now well known. The article "IEE transaction on Consumer Electronics 473-479" explains the benefit and the working principle of power supplies of this type. These power supplies include a transformer having primary windings and secondary windings.
The invention relates to the architecture of the transformer. A known example of a transformer of this type for a switched-mode power supply is described in European Patent EP 71008, filed in the name of Licentia Patent Wervaltung.
This patent describes a transformer for a switched-mode power supply, in which the windings of the primary and of the secondary are not produced in successive layers around a ferromagnetic core, but in adjacent chambers arranged axially along the core. Each chamber is separated from the next by a sheet of insulating material. A transformer of this type is represented in FIG. 1 under the general reference 1.
It has a ferrite core 2 formed by two E-shaped ferrite half-cores 3, 4 which are bonded to one another by a line of adhesive 5 in order to form a right-angled torus with rectangular cross-section to which a central part, masked in FIG. 1 by a former 6, is adjoined; the former 6 is partially represented in FIG. 2. It has a cylindrically shaped hollow central core 7 and walls 8 perpendicular to this body. Only one of these walls 8 has been represented in FIG. 2. Windings (not shown) are present in chambers 9 formed between two walls. The wires 10, 10' of these windings are connected to connection pins 11, 11'. The primary windings are connected to the pins 11, and the secondary windings to the pins 11'. All the primary connection pins 11 are located on one side of the transformer. All the secondary connection pins 11' are located on the other side of the transformer. Because of the perspective represented in FIG. 1, only the primary winding wires 10 and the primary connection pins 11 are represented. The secondary connection wires 10' and the secondary connection pins 11' are all, for good isolation, located on a different side of the transformer. The wires 10' of the secondary are connected to the pins 11' of the secondary in a technical fashion similar to the connection of the wires 10 of the primary to the pins 11 of the primary. The term "other side of the transformer" means that the primary pins 11 and the secondary pins 11' lie on either side of a plane of symmetry of the core. In the case represented in FIG. 1, this is the plane of symmetry common to all three branches of the Es forming each half-core 3, 4 of the core 2.
The term "ends 12, 12'" of the primary wires 10 and secondary wires 10' is hereafter used to denote the portion of wire 10, 10' located between the end of a winding and a connection pin 11 or 11'. These ends are held in place by notches 13, 13' formed on one side of the insulating walls 8. The notches 13 or 13' of a wall 8 together form a connection comb 14 of the wall 8.
The combs 13 of the walls 8 guiding the primary wires 10 are located on the same side as the primary connection pins 11. The combs 13' of the walls 8 guiding the secondary wires 10' are located on the same side of the secondary connection pins 11'.
This transformer architecture permits good isolation of the primary side, also referred to as the "hot side", from the secondary side, also referred to as the "cold side". This good isolation is due to the fact that the primary and secondary windings are in chambers 9 which are DC isolated from one another by insulating walls 8 and to the fact that the primary pins 11 and secondary pins 11' are remote from one another.
The leakage inductances remain acceptable because chambers 9 containing primary windings and chambers 9' containing secondary windings have alternate axial positions, and there is a fairly large number of them. However, all other things being equal, this leakage inductance increases when the switching frequency increases.